The present invention relates to a new and improved casting method and, more specifically, to a casting method in which a mold structure is preheated in a furnace chamber, molten metal is poured into the preheated mold structure, and the mold structure is withdrawn from the furnace chamber.
During the manufacture of superalloy castings, it is a common practice to place a ceramic mold on a chill plate made of copper or other heat conductive material. The ceramic mold is preheated in an evacuated furnace chamber surrounded by an induction coil and graphite susceptor which forms the inner wall of the furnace chamber. During the casting process, temperatures can get as high as 3,000 degrees F. These high temperatures, coupled with the vacuum, can cause decomposition of the binder system used in the mold. In addition, these high temperatures can cause components of the furnace, such as the graphite susceptor, insulating materials between the susceptor and the induction coil, and insulating material that covers the top of the induction coil, to decompose. The decomposed materials, including gases, move into the mold where they are deposited and/or trapped. Upon the introduction of molten metal into the mold, the molten metal reacts with the trapped gases and/or deposited material, altering the chemistry of the molten metal. The contaminants become embedded in the castings and can result in the castings being rejected.
It has previously been suggested that the accumulation of contaminants in a mold could be retarded by providing a hollow cover for an opening in an upper side wall of the furnace chamber. It is believed that the contaminants tend to become deposited or condensed on surfaces inside the cover rather than in the mold. This method of avoiding the accumulation of contaminants in a mold is described in U.S. Pat. No. 3,385,346, issued May 28, 1968 and entitled "Method and Apparatus for Removal of Condensed Deposits From Mold Covers".